The present invention relates generally to crystal growing apparatus used in growing monocrystalline silicon ingots, and more particularly to a heater assembly for use in such a crystal growing apparatus.
Single crystal silicon, which is the starting material for most semiconductor electronic component fabrication, is commonly prepared by the so-called Czochralski (xe2x80x9cCzxe2x80x9d) method. The growth of the crystal is most commonly carried out in a crystal pulling furnace. In this method, polycrystalline silicon (xe2x80x9cpolysiliconxe2x80x9d) is charged to a crucible and melted by a heater surrounding the outer surface of the crucible side wall. A seed crystal is brought into contact with the molten silicon and a single crystal ingot is grown by slow extraction via a crystal puller. After formation of a neck is complete, the diameter of the crystal ingot is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process, the crystal diameter must be reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the pull rate and heat supplied to the crucible. When the diameter becomes small enough, the ingot is then separated from the melt.
It is now recognized that a number of defects in single crystal silicon form in the growth chamber as the ingot cools from the temperature of solidification. More specifically, as the ingot cools intrinsic point defects, such as crystal lattice vacancies or silicon self-interstitials, remain soluble in the silicon lattice until some threshold temperature is reached, below which the given concentration of intrinsic point defects becomes critically supersaturated. Upon cooling to below this threshold temperature, a reaction or agglomeration event occurs, resulting in the formation of agglomerated intrinsic point defects.
The type and initial concentration of these point defects in the silicon are determined as the ingot cools from the temperature of solidification (i.e., about 1410xc2x0 C.) to a temperature greater than about 1300xc2x0 C. (i.e., about 1325xc2x0 C., 1350xc2x0 C. or more); that is, the type and initial concentration of these defects are controlled by the ratio v/G0, where v is the growth velocity and G0 is the average axial temperature gradient over this temperature range. Accordingly, process conditions, such as growth rate (which affect v), as well as hot zone configurations (which affect G0), can be controlled to determine whether the intrinsic point defects within the single crystal silicon will be predominantly vacancies (where v/G0 is generally greater than the critical value) or self-interstitials (where v/G0 is generally less than the critical value).
Defects associated with the agglomeration of crystal lattice vacancies, or vacancy intrinsic point defects, include such observable crystal defects as D-defects, Flow Pattern Defects (FPDs), Gate Oxide Integrity (GOI) Defects, Crystal Originated Particle (COP) Defects, and crystal originated Light Point Defects (LPDs), as well as certain classes of bulk defects observed by infrared light scattering techniques (such as Scanning Infrared Microscopy and Laser Scanning Tomography).
Also present in regions of excess vacancies, or regions where some concentration of free vacancies are present but where agglomeration has not occurred, are defects which act as the nuclei for the formation of oxidation induced stacking faults (OISF). It is speculated that this particular defect, generally formed proximate the boundary between interstitial and vacancy dominated material, is a high temperature nucleated oxygen precipitate catalyzed by the presence of excess vacancies; that is, it is speculated that this defect results from an interaction between oxygen and xe2x80x9cfreexe2x80x9d vacancies in a region near the V/I boundary.
Defects relating to self-interstitials are less well studied. They are generally regarded as being low densities of interstitial-type dislocation loops or networks. Such defects are not responsible for gate oxide integrity failures, an important wafer performance criterion, but they are widely recognized to be the cause of other types of device failures usually associated with current leakage problems.
Agglomerated defect formation generally occurs in two steps; first, defect xe2x80x9cnucleationxe2x80x9d occurs, which is the result of the intrinsic point defects being supersaturated at a given temperature. Once this xe2x80x9cnucleation thresholdxe2x80x9d temperature is reached, intrinsic point defects agglomerate. The intrinsic point defects will continue to diffuse through the silicon lattice as long as the temperature of the portion of the ingot in which they are present remains above a second threshold temperature (i.e., a xe2x80x9cdiffusivity thresholdxe2x80x9d), below which intrinsic point defects are no longer mobile within commercially practical periods of time. While the ingot remains above this temperature, vacancy or interstitial intrinsic point defects diffuse through the crystal lattice to sites where agglomerated vacancy defects or interstitial defects, respectively, are already present, causing a given agglomerated defect to grow in size. Growth occurs because these agglomerated defect sites essentially act as xe2x80x9csinks,xe2x80x9d attracting and collecting intrinsic point defects because of the more favorable energy state of the agglomeration.
Accordingly, the formation and size of agglomerated defects are dependent upon the growth conditions, including v/G0 (which impacts the initial concentration of such point defects), as well as the cooling rate or residence time of the main body of the ingot over the range of temperatures bound by the xe2x80x9cnucleation thresholdxe2x80x9d at the upper end and the xe2x80x9cdiffusivity thresholdxe2x80x9d (which impacts the size and density of such defects) at the lower end. Thus, control of the cooling rate or residence time enables the formation of agglomerated intrinsic point defects to be suppressed over much larger ranges of values for v/G0; that is, controlled cooling allows for a much larger xe2x80x9cwindowxe2x80x9d of acceptable v/G0 values to be employed while still enabling the growth of substantially defect-free silicon.
As an example, one crystal puller used for controlling the cooling of monocrystalline ingots above the nucleation threshold of intrinsic point defects is disclosed in co-assigned U.S. patent application Ser. Nos. 09/344,003 and 09/338,826, which are incorporated in their entirety herein by reference. The crystal puller includes an electrical resistance heater mounted in the pull chamber of the crystal puller housing generally toward the bottom of the pull chamber of the housing. The electrical resistance heater has heating segments that may be constructed of equal length (e.g., a non-profiled heater) or of stepped, or staggered lengths (e.g., a profiled heater). As portions of the ingot grown in the puller are pulled upward into radial registration with the heater, heat is radiated by the heater to these portions of the ingot to reduce the cooling rate of the ingot.
Co-assigned U.S. patent application No. 09/661,745, the full disclosure of which is incorporated herein by reference, discloses a quenching process for growing a monocrystalline silicon ingot according to the Czochralski method in which the nucleation and/or growth of interstitial type defects is suppressed by controlling the cooling rate of the ingot through nucleation. More particularly, initial growth conditions are selected to provide an ingot containing silicon self-interstitials as the predominant intrinsic point defect from the center to the edge of the ingot, or a central core in which vacancies are the predominant intrinsic point defect surrounded by an axially symmetric region in which silicon self-interstitials are the predominant intrinsic point defect. As the ingot cools while being pulled upward within the crystal puller, the temperature of the ingot is maintained above the temperature range at which nucleation of the self-interstitials occurs, such as about 850xc2x0 C.-950xc2x0 C., for a time period sufficient for adequate diffusion of intrinsic point defects. Then, the ingot is rapidly cooled, or quenched, through the nucleation temperature range to inhibit nucleation. Below the nucleation temperature range, no further nucleation will occur. The process is disclosed as producing ingots that are substantially free of intrinsic point defects.
While the crystal puller configurations discussed above as having an electrical resistance heater disposed in the pull chamber of the crystal puller are effective for increasing the dwell time of the ingot above a desired temperature, these configurations are not as efficient as desired for carrying out the quenching process described above to produce ingots that are substantially free of intrinsic point defects. More particularly, the electrical resistance heaters previously disclosed are not long enough to grow defect-free ingots of substantial length, such as about one meter. However, lengthening the heater, such as to provide a single, elongate electrical resistance heater disposed in the pull chamber would require a substantial amount of electrical power to achieve the desired heating power output. Also, using such a heater, even if profiled in construction, substantially limits the flexibility of profiling the heating power output needed to control the temperature of the ingot as it is pulled upward within the crystal puller. Using such a heater would also limit the ability to grow ingots of various lengths in the crystal puller without changing the size of the heater.
Among the several objects and features of the present invention may be noted the provision of a crystal puller and method for growing an ingot according to the Czhochralski method which facilitates the growth of ingots that are substantially free of agglomerated intrinsic point defects; the provision of such a crystal puller that facilitates the growth of ingots of various lengths without changing the crystal puller configuration; the provision of a heater assembly which increases the duration at which the ingot resides at a temperature above the nucleation temperature of intrinsic point defects; the provision of such a heater assembly which allows for various heating power output profiles; and the provision of an adapter which facilitates the adaptation of existing crystal pullers to incorporate such a heater assembly.
Generally, a crystal puller of the present invention for growing monocrystalline silicon ingots according to the Czochralski method generally comprises a housing, a crucible in the housing for containing molten silicon and a pulling mechanism for pulling a growing ingot upward from the molten silicon. A first electrical resistance heater comprising a first heating element is sized and shaped for placement in the housing of the crystal puller generally above the crucible in spaced relationship with the outer surface of the growing ingot for radiating heat to the ingot as it is pulled upward in the housing relative to the molten silicon. The first heating element has an upper end and a lower end, with the lower end of the first heating element being disposed substantially closer to the molten silicon than the upper end when the first heating element is placed in the housing. A second electrical resistance heater comprising a second heating element is sized and shaped for placement in the housing of the crystal puller in generally longitudinally spaced relationship above the first electrical resistance heater and in spaced relationship with the outer surface of the growing ingot for radiating heat to the ingot as it is pulled upward in the housing relative to the first electrical resistance heater. The second heating element has an upper end and a lower end, with the lower end of the second heating element being in closely spaced relationship with the upper end of the first heating element of the first electrical resistance heater. The crystal puller is substantially free of structure interposed between the lower end of the second heating element and the upper end of the first heating element.
In another embodiment, the crystal puller comprises a housing, a crucible in the housing for containing molten silicon and a pulling mechanism for pulling a growing ingot upward from the molten silicon. A first electrical resistance heater comprising a first heating element is sized and shaped for placement in the housing of the crystal puller generally above the crucible in spaced relationship with the outer surface of the growing ingot for radiating heat to the ingot as it is pulled upward in the housing relative to the molten silicon. The first heating element having an upper end and a lower end, with the lower end of the first heating element being disposed substantially closer to the molten silicon than the upper end when the first heating element is placed in the housing. A second electrical resistance heater comprising a second heating element is sized and shaped for placement in the housing of the crystal puller in generally longitudinally spaced relationship above the first electrical resistance heater and in spaced relationship with the outer surface of the growing ingot for radiating heat to the ingot as it is pulled upward in the housing relative to the first electrical resistance heater. The second heating element has an upper end and a lower end, with the lower end of the second heating element being in spaced relationship with the upper end of the first heating element. At least one of the first and second electrical resistance heaters is constructed such that the heating power output generated by the heating element of the at least one of the first and second electrical resistance heaters gradually increases from the lower end to the upper end of the heating element.
A combination adapter and heater assembly of the present invention for use with a crystal puller of the type used for growing monocrystalline silicon ingots according to the Czochralski method generally comprises a generally tubular adapter having a side wall defining a central passage sized for allowing through passage of the ingot as the ingot is pulled upward within the housing. A flange extends generally radially outward from the side wall for releasably mounting the adapter on the housing above the crucible whereby the side wall of the adapter at least partially defines the housing. A heater assembly is disposed in the adapter and mounted on the adapter side wall such that the adapter and heater assembly are installable in and removable from the crystal puller as a single unit. The heater assembly includes a first electrical resistance heater comprising a first heating element mounted on the adapter side wall for placement in the housing of the crystal puller generally above the crucible when the adapter and heater assembly are installed in the crystal puller. The first heating element has an upper end and a lower end, with the lower end of the first heating element being disposed substantially closer to the molten silicon than the upper end when the adapter and heater assembly are installed in the crystal puller. The heater assembly also includes a second electrical resistance heater comprising a second heating element mounted on the adapter side wall above the first electrical resistance heater. The second heating element has an upper end and a lower end, with the lower end of the second heating element being in closely spaced relationship with the upper end of the first heating element.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.